You'll have to protect the boxwood from wind during winter. They are hardy, but they are not as hardy as Junipers in terms of cold weather. I live in Vermont, USA and have a few green velvet boxwoods being trained for Bonsai. Where I live (northern VT) is considered zone 3b (no colder than −35 °F (−37 °C)), and if I want my potted trees to survive,. the safest bet is to build a cold frame, which I am doing soon. Maybe you can consider that too? making a small cold frame for just 1 tree should be a piece of cake.